In a time-division multiplexed communication system serving many subscribers, the transmission and reception of messages at each terminal can be accomplished by the use of a variety of means including repeaters, directional couplers, taps and switches. Among the desirable properties of any such device are good coupling efficiency, low insertion loss, and transparency to the main stream message under failure of the terminal. Repeaters are generally more expensive and band limiting, and lack a built-in fail safe mechanism. Taps, on the other hand, have increasing insertion loss with increasing coupling efficiency and are mode dependent, while mode independent beam splitters tend to have higher insertion losses (&gt;1.5 dB).
One can use planar electrooptic waveguide switches of the type disclosed, for example, in U.S. Pat. Nos. 4,003,629; 4,008,947 and 4,130,342. However, such devices tend to be single mode devices and at the present level of development exhibit losses of the order of 8 dB. In addition, the operating voltages for waveguide switches are much too high for fast switching applications, since fast, high voltage word generators are not commonly available.
In a paper entitled "Fiber-Optic Switching with Liquid Crystals" by R. H. Soref, published in the April, 1979 issue of the Proceedings of the Society for the Photo Optical Instrumentation Engineers, a switch is described which relies upon total or partial optical reflection by a liquid crystal layer in response to an applied electrical signal. Such devices, however, are not particularly efficient in that they tend to reflect a significant percentage of the input power in the transmitting mode. In addition, the switching frequency of such devices is very low. In this reference, the switching rate given is less than 500 Hz.